Why:
* Ember and Ember-Data will move to JavaScript modules allowing us to
only import functions we actually use.
This PR:
* Proposes creating local constants for functions on `Ember` and `DS`
* This means when these libraries are converted to modules, updating the
code is as simple as converting:
```js
const { computed } = Ember;
```
to:
```js
import { computed } from 'ember';
```